Integrated service digital network (ISDN) technology and digital subscriber line (XDSL) technology provide higher bandwidth service for the transmission of voice, video and data over existing twisted pair telephone lines. ISDN and XDSL services may be combined on one twisted pair line but some problems may occur. ISDN typically transmits signals using frequencies approximately between 4 KHz and 80 KHz while the ADSL standard specifies that signals are to be transmitted approximately between 25 KHz and 1.1 MHz or higher. The overlap in frequency may be solved by transmitting ISDN signals at their usual frequencies and restricting XDSL signals to a higher frequency band. Due to the spectral proximity of ISDN and XDSL signals, however, mutual interference may still occur.